I'm working on a little model to get me into the hang of using ZBrush with Maya, but I am having a problem with displacement maps looking rubbish - but only sometimes.

Here all the maps are applied to the tree-balls in the same way, but you can just see that little one to the back looks really bitty, and this can happen to all of them, but depending on where I put the camera and stuff. If I fiddle around with the sample count under tesselation, I can make it go away, but then I move the camera and it comes back again. Feature displacement is off. Anyone have any idea what the problem is?